In modern, highly supercharged Otto-cycle internal combustion engines, the phenomena of premature firing and spontaneous ignition are increasingly encountered with increasing charge and peak pressures. These phenomena occur primarily at high loads throughout the engine speed range and are distinguished by uncontrolled spontaneous firing of the fuel-air mixture in the combustion chamber at a time before the spark ignition by means of the ignition plug. The result is uncontrolled firing and combustion with extreme peak pressures, very high combustion temperatures, high pressure amplitudes and thus a damaging action.
Whereas premature firing normally occurs as a stochastic individual event owing to incandescent residual particles remaining in the combustion chamber and disappears again after a certain number of working cycles, spontaneous ignitions are self-intensifying processes of irregular combustion prior to the regular ignition time. Triggers here are hot surfaces and superheated components such as, for example, electrodes of ignition plugs, outlet valves, sharp superheated edges in the combustion chamber, or soot and fuel deposits on hot combustion chamber walls. Spontaneous firing occurring at an early time causes the pressure and the temperature in the combustion chamber to increase even further, and the triggering component or the surface is heated yet further. During the next cycle, the spontaneous ignition occurs even earlier, and the combustion chamber temperature increases further. In the end phase, in which the spontaneous ignition reaches its steady state, the self-ignition time is so early that knocking can no longer be identified. The spontaneous ignition cannot be prevented by retarding the ignition angle.
The high thermal load of the combustion chamber can result in damage or even total destruction of the piston, or in melting of the electrodes of the ignition plug or of parts of the highly loaded outlet valves. To prevent damage to the internal combustion engine, early identification of such spontaneous ignitions is necessary.
DE 10 2007 024 415 B3 describes a method for the identification of a spontaneous ignition of an applied-ignition internal combustion engine having at least one cylinder, which is connected to a crankshaft. In the method, a rotational speed of the crankshaft is measured during a compression stroke of the cylinder during a first time interval in the working cycle of the internal combustion engine. Furthermore, a knocking signal is detected during a working stroke of the cylinder during a second time interval in the working cycle of the internal combustion engine. Spontaneous ignition of the cylinder is identified if the rotational speed of the crankshaft is slowed in relation to a comparative value and knocking combustion is identified on the basis of the knocking signal.
DE 10 2012 221 249 B3 presents a method and a device for identification of a spontaneous ignition in an applied-ignition internal combustion engine having at least one cylinder which, together with a piston, delimits a combustion chamber, having a combustion chamber pressure sensor for measuring the pressure in the combustion chamber, having a crankshaft angle sensor which outputs a signal representative of the crankshaft angle, and having a control device for the control and/or regulation of the internal combustion engine. During a compression stroke of the internal combustion engine, values for the combustion chamber pressure are detected at defined crankshaft angles within an evaluation window, a filtered pressure value is determined from the detected values for the combustion chamber pressure, and theoretical pressure values in the combustion chamber, such as would arise if no combustion were to take place in the combustion chamber, are determined at the defined crankshaft angles. The value of the pressure difference between the filtered pressure value and the theoretical pressure values is formed, the value of the pressure difference is compared with a predefined threshold value, and in the event of the threshold value being exceeded, a spontaneous ignition in the combustion chamber is inferred.
From DE 198 59 310 A1, it is known for premature firing events to be identified through detection of the ion flow, wherein here, the use of a regulation loop is described in which operating parameters of the internal combustion engine are regulated by means of the engine controller on the basis of identified premature firing events. It is proposed that, in the event of the measured ion flow exceeding a threshold value, the ignition angle be shifted in regulated fashion toward later ignition times, and a preset ignition angle be reverted to again after elimination of the premature firing.